A typical automotive-type solid electrolyte exhaust gas oxygen sensor is disclosed in U.S. Pat. No. 3,844,920 Burgett et al. It has a zirconia sensing element shaped as a tapered thimble. One end is open and has a circumferential flange. The other end is closed, and forms the most active part of the element. The interior and exterior of the thimble have separate porous electrode coatings of platinum, palladium, or the like. The inner electrode is exposed to a source of oxygen, such as air or a mixed metal oxide, for establishing a reference potential. This electrode has generally been formed by painting a coating of a platinum ink onto the zirconia thimble, drying the coating, and then firing the coated thimble at an elevated temperature. An improved technique by which it can be applied is described and claimed in U.S. patent application Ser. No. 080,449 entitled "Reference Electrode Printing Process and Mask for Exhaust Gas Oxygen Sensor," filed on Oct. 1, 1979 in the name of John Trevorrow and assigned to the assignee of this invention, now U.S. Pat. No. 4,264,647.
The outer electrode is exposed to the exhaust gas for establishing a potential determined by exhaust gas oxygen concentration. The outer electrode can be a porous precious metal thick film, like the inner electrode. However, it is preferred that this outer electrode be a thin film, applied by evaporation, sputtering, chemical vapor deposition or other such thin film deposition techniques. On the other hand, it has been difficult to consistently reproduce desirable properties, such as porosity and electrical parameters, in the thin film electrodes. As a result, yields of satisfactory electrode properties have been limited, and various ancillary procedures have been developed to improve them. For example, U.S. Pat. No. 3,978,006 Topp et al. discloses heating the solid electrolyte body after electrode deposition, to form pores in the electrode coating if it is not porous as deposited. U.S. Pat. No. 4,136,000 Davis et al. discloses treating the electroded sensor element chemically and electrolytically to enhance sensor properties. Moreover, it is known that zirconia-type exhaust gas sensors, particularly those with a sputtered exhaust gas electrode, are likely to change electrical characteristics after a short time in operation. Generally the change is an improvement, such as a reduction in switching response time. Consequently, it has been proposed to operate such sensors functionally in an actual or simulated exhaust gas stream until they are sufficiently stablized, before installing them in an actual working system. Such treatments, of course, add to the cost of manufacture. Moreover, the yield of higher performance sensors is still inherently limited by the quality of the electrode film originally deposited.
We have found how to sputter palladium and palladium-platinum films onto the zirconia surface in such a manner that the film is consistently porous as deposited and has a consistently high surface area as deposited, which contributes to a greater yield of high quality sensors. Sensors with low lean-to-rich switching response times are produced, without post-electroding treatments. Palladium electrodes made in accordance with this invention consistently do not appear to exhibit any significant initial use changes, which have been referred to as break-in or aging effects. For example, pure palladium electrodes sputtered by this invention can exhibit rich-to-lean switching response times that are initially about as low as the lean-to-rich switching response times. Hence, palladium electrodes produced by our process have more symmetrical switching response times, as formed. With a significant proportion of platinum included in the electrode, the rich-to-lean switching response of the electrode made by this invention may be low, as formed, but not as low as the lean-to-rich switching response until after a short actual or simulated aging. Hence, a high yield of significantly fast sensors is obtained, as formed, or with only minimal aging. In fact, if sensors having exhaust electrodes produced in accordance with this invention are susceptible to aging, it can probably be done by a simple furnace treatment, as is disclosed and claimed in U.S. patent application Ser. No. 30,747 entitled "Aging Treatment for Exhaust Gas Oxygen Sensor," filed on April 17, 1979 in the names of Morris Berg, Slater W. Hawes, Frederick L. Kennard, III and Paul C. Kikuchi and assigned to the assignee hereof, now U.S. Pat. No. 4,253,934.